Yes, you should be fine. I had no issues in my last truck('98 GMC 1500 Z71) running it. As far as the LS additive, mine didn't need it, but I don't recall specifically which rear end was in it. The manual specs call for conventional in the front, but if you want to spend the extra $ go for it.
I run shell coventional 80W-90 gear oil in my 98 Z71 but used to run Mobil 1 in both diffs. It might just be a coincidence but I had to have my rear end rebuilt at 75,000 miles with hardly any towing or hauling. After that I went back to conventional and now at 120,000 miles, no problems. I first started hearing noises at about 55,000 miles but could not figure out what it was and finally had it looked at professionally which ended up with the repair. Others have had good luck with it so for that price you might as well try it.